The present invention pertains to a system and a method for controlling a digital sensor.
A sensor is a system which transforms an observed first physical quantity into a usable second physical quantity, such as an electric voltage, a height of mercury, an electric current intensity, a needle deviation, etc. The sensor is an interface between a physical process and a manipulatable item of information.
The sensor must be differentiated from the measurement apparatus, which is an autonomous apparatus, having a display or a device for storing data, which is not necessarily the case for the sensor. Sensors are the basic elements of data acquisition systems.
Sensors may be classed in various ways.
For example, they may be distinguished by their energy consumption, as passive sensors or active sensors.
A passive sensor is a sensor which does not require any energy intake from outside in order to operate, for example a thermistor, a potentiometer, or a mercury thermometer. A passive sensor can be modeled by an impedance. A variation in the measured physical phenomenon produces a variation in the impedance. An active sensor is, on the other hand, a sensor which requires energy intake from outside in order to operate, for example a mechanical chronometer, an extensometry gauge, also called a strain gauge, or a gyrometer. An active sensor can be modeled by generators such as photovoltaic and electromagnetic devices. Thus, an active sensor generates either an electric current, or an electric voltage as a function of the intensity of the physical phenomenon measured.
Sensors can also form the subject of a classification by type of output, into analog sensors and digital sensors.
The output signal of an analog sensor is an analog signal, such as an electric voltage or an electric current, while the output signal of a digital sensor is a digital signal, such as a train of pulses, a binary digital code or a fieldbus.
A sensor is generally characterized by several criteria such as the measured physical quantity, the operating range, or the precision. To use a sensor under the best conditions, it is often useful to undertake a calibration and to ascertain the uncertainties in measurements relating thereto.
A digital sensor comprises a transducer delivering as output an analog signal representative of the measured physical quantity. A transducer is a device transforming one physical quantity into another, i.e. an analog signal such as an electric voltage is obtained at the output of the transducer. In a conventional manner, digital sensors also comprise a module for implementing gain and/or shift on the analog output signal of said transducer, as well as an analog-digital converter at the output of the sensor.
The module for implementing gain and/or shift on the analog output signal of the transducer makes it possible to be able to use a digital sensor in an environment, such as a housing, in which an external disturbance modifies the physical quantity to be measured in a relatively constant manner. This may, for example, be the case for a disturbance of a magnetic sensor integrated into a portable telephone comprising loudspeakers furnished with powerful magnets.
In the case of a digital sensor, for example magnetic, illustrated in FIG. 1, the undisturbed nominal operation of which is represented in FIG. 2, when it is integrated into a carrier device consisting of amagnetic components, and the nominal operation of which, when it is integrated into a carrier device in which magnetic sources are embedded, such as a portable telephone, is illustrated in FIG. 3, in which these magnetic sources displace the operating point of the sensor in a significant but known or identifiable manner during the manufacture of the sensor or at diverse times in its duration of use.
In the subsequent description, the examples illustrate cases in which the digital sensor is a magnetometer, but the invention applies to any type of digital sensor.
In FIG. 1, the digital sensor CN, in this instance a digital magnetometer, comprises a transducer TRD delivering as output an analog signal SA1 representative of the measured physical quantity GP. In this instance, the analog signal SA1 at the output of the transducer is an electric voltage. The digital magnetometer CN also comprises a module MGD for implementing gain and/or shift on the analog output signal SA1 of said transducer, so as to transmit an analog signal SA2 to an analog-digital converter CAN at the output of the sensor, so as to deliver a digital signal SN at the output of the digital sensor CN.
FIG. 2 illustrates the undisturbed nominal operation of the digital magnetometer CN of FIG. 1, for example when it is included in an amagnetic housing.
The measured physical quantity GP, in this instance the magnetic field, is the ambient magnetic field, undisturbed by an additional magnetic field. The analog signal GP representing the physical quantity is modified by the transducer TRD into an analog signal SA1, in this instance an electric voltage, to which a gain and/or shift is applied by the module MGD for implementing gain and/or shift, predetermined, depending on the hardware used, so as to obtain an analog signal SA2 included in a range of values acceptable by the analog/digital converter CAN. The range of measurements of the physical quantity GP usable by the transducer TRD, in this instance the magnetic field, comprises values for example between −150 μT and 150 μT, this corresponding as equivalence at the output of the module MGD for implementing gain and/or shift and at the input of the analog/digital converter CAN to an electric voltage SA2 included in a range of values for example from 150 mV to 500 mV. At the output of the analog/digital converter CAN, the corresponding digital signal SN, coded for example on one byte, or stated otherwise on 8 bits of data, can take integer values lying between 0 and 256.
FIG. 3 illustrates the disturbed nominal operation of the digital magnetometer CN of FIG. 1, for example when it is included in a portable telephone comprising loudspeakers creating a predetermined magnetic disturbance.
The measured physical quantity GP, in this instance the magnetic field, is the ambient magnetic field, disturbed by an additional magnetic field due to components of the portable telephone in which the digital sensor CN is embedded. With respect to the case of FIG. 2, the digital sensor being subjected to a constant additional magnetic field due to elements of the portable telephone, such as loudspeakers, the module MGD for implementing gain and/or shift, furthermore applies a constant additional shift to the signal GP, substantially equal to the opposite of the additional magnetic field, so that the analog signal at the input of the digital sensor CN is shifted in such a way as to remain in the range of measurements of the digital sensor CN, i.e. the transducer TRD.
Digital sensors may, however, saturate because of a phenomenon external to that of the measurement, causing, by surprise, the physical quantity to be measured to depart from the measurement range of the sensor.